When recording information to optical media, such as write once or rewritable optical disks, it is important to detect whether a particular sector has already been recorded. For example, a host device to which an optical drive is attached can issue a command to locate the next available blank sector to start a write operation. If an error occurs in locating this sector, a blank sector can remain unwritten or a written sector with previously written information can be inadvertently overwritten and the previously written information lost. Moreover, even if the overwritten sector did not contain valuable customer data, the overwrite can make it difficult for the drive's servo to track through the overwritten sector and make it difficult or impossible to read the remaining sectors on the track. Similarly, the drive may be required to locate blank sectors as part of data recovery procedures. It will be appreciated that serious data integrity problems can occur when written and blank sectors cannot be reliably distinguished.
With re-writable optical media, such as magneto-optical (MO), one or more directories are kept on the media identifying the sectors which have been recorded. However, if a sector becomes unreadable, it is necessary to verify its status other than through the directory. With write once media, such as write once read many (WORM), directories are not kept on the media due to the amount of space required. Therefore, another method of determining whether a sector is written is necessary.
Prior to the present invention, one of a number of possible parameters was measured while reading the data area of a sector and compared to a fixed value in order to determine whether the sector was written. Such a procedure can be inconsistent and unreliable, however, due to factors which can vary from drive to drive and from disk to disk. These factors include, for example, drive to drive electronic variations (as from heat, age and component variations within a tolerance range), media to media amplitude variations, optical path variations (including variations caused by dust and other debris on optical elements), different read power levels suggested by different media manufacturers, and variations in media reflectivity. More specifically, it will be appreciated that the readback amplitude on a population of drives and media will vary greatly. Media variation alone can cause variation as great as 10:1 in a multi-function drive which detects ablative, phase change, and magneto-optic disks. Drive tolerances can add another 2:1. Because of this, the readback amplitude at a given fixed gain level will vary wildly. A setting which is insufficient to read ID's on one drive media combination may cause the next to saturate. For this reason, the threshold calibration routine must be adaptive to the degree that it allows modification of the channel gain settings and re-calibration of the envelope threshold if the gain setting present is inadequate for either processing ID's or sufficient resolution to set the threshold.